MXR phaser

[Harry Bissell]

If you mean the MXR phase 90 or 100... it is not a BBD
(or else I wouldn't have one... hahahah!)
It uses a phase shift stage that has an op-amp (looks like a diff amp with the
resistor going to the non-inverting input replaced with a cap, and a FET where the
resistor from the non-inverting to ground should be. The FET sets the frequency
where the stage shifts phase. It is also known as an all-pass filter.

You don't get the "Phase" sound until you add or subtract the phase shift signal
with the original.

The Phase 90 has 4 stages in series, the phase 100 has (i think) 10 stages, with
feedback options selected by a switch.

There are a few tricks in the Phase 90... there is a zener diode (about 4.5 volts)
The Cathode of that diode (for all intents...) is the "ground"  stick your scope or
meter there.
There is a trim pot inside that sets the bias on the fets... if that is set wrong...
no shift to the sound. Even if one fet is dead the unit should still work... or two,
or three... but the sound will get worse each time.

I'd check out that 75K pot that sets the speed... make sure its good...

I think "marjan" has the schems posted... if not try GEO web page or R. Keen guitar
effects..

H^)    harry

Brian.Dekok at ca.jdsunph.com wrote:

> Speaking of the MXR phaser, I have one at home that's not working. The sound
> changes slightly when the button is switched, but I think the LFO isn't sweeping
> it. Can someone tell me of a similar transistor to the C1849?  Also, does this
> pedal use BBD's? The chips in it are TI 72L044's. Any help would be appreciated.
>
> Don Tillman <don at till.com> on 01/28/2000 04:24:07 AM
>
> To:   WeAreAs1 at aol.com
> cc:   farky at ix.netcom.com, synth-diy at mailhost.bpa.nl (bcc: Brian Dekok/HQ/JDS
>       FITEL Inc)
> Subject:  Re: vibrato
>
>    From: WeAreAs1 at aol.com
>    Date: Thu, 27 Jan 2000 23:55:41 EST
>
>    farky at ix.netcom.com wrote:
>    << Does anybody know of a simple circuit to produce vibrato on an input
>    audio signal?  The vibrato I'm speaking of is the pitch-shift kind, not
>    to be confused with tremolo. >>
>
>    You can use an analog delay chip (BBD) and modulate its clock with an LFO.
>
> Another approach is to use a phase shifter circuit without the
> original signal mixed into the phase shift output.  That means you can
> take a standard phase shifter stomp box, like the ol' MXR Phase 90,
> and clip one connection, and it's a vibrato.  Note that the effect can
> be pretty subtle at low sweep speeds.
>
>  -- Don